This invention relates to an apparatus for controlling the energization of electrically heated vehicle windows.
An example of such a vehicle window heating apparatus is disclosed in Japanese Patent Kokai No. 61-33735. The apparatus involves a heating element provided for de-icing the vehicle window. The heating element is powered by a generator which has a main output connected to charge a vehicle battery and a field winding fed from a voltage regulator. A control circuit is provided for connecting the generator main output to the heating element while disconnecting it from the vehicle battery and the voltage regulator so that the full generator output is delivered to energize the heating element for a limited period of time when a window de-icing operation is required. This is effective to ensure that the heating element is supplied with a large amount of power required for de-icing the vehicle window.
One disadvantage with such a conventional apparatus is that high-voltage power will be supplied to damage the heating element and shorten its service life when the window de-icing operation is required during vehicle running conditions. This is because the generator field winding is driven at a speed proportional to the engine speed, the heating element is supplied with a voltage determined by the engine speed during the window de-icing operation where the voltage regulator is disconnected from the generator main output. Another disadvantage is that the vehicle battery may die during vehicle running conditions since the vehicle battery is disconnected from the generator main output during the window de-icing operation.